This my Elastix SIP trunk configuration for a successful connection to Internode. I'm currently using the NodePhone2-Starter plan for testing but it should be the same for all higher plans. I force all extensions and the trunk to use the G711u codec. You'll need to specify your own username and password/secret.

I called my trunk Internode, specified my outbound CID and the number of channels I have available in my main Trunk preferences then used the following settings.

Setting context=from-trunk-sip-Internode allows your inbound route to take the number of current outbound calls into account when deciding whether there are enough channels availableif you have specified your maximum channels for this SIP trunk, which for my plan is 2.